Molecular sieves are a commercially important class of crystalline materials having distinct crystal structures with ordered pore structures and characteristic X-ray diffraction patterns. Natural and synthetic crystalline molecular sieves are useful as catalysts and adsorbents.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association (IZA) according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the “Atlas of Zeolite Framework Types,” Sixth Revised Edition, Elsevier, 2007.
One known molecular sieve for which a structure has been established is the material designated as LTL. LTL framework type materials have a framework comprising cancrinite-type cages linked by double six rings in columns and cross-linked by single oxygen bridges to form 12-membered ring pore channels. Examples of LTL framework type molecular sieves include Linde Type L (zeolite L) and LZ-212. LTL framework type materials are of significant commercial interest because of their activity as catalysts in hydrocarbon conversion reactions, particularly aromatization reactions (see, e.g., P. W. Tamm et al., Stud. Surf. Sci. Catal., 1988, 38, 335-353).
Zeolite L is disclosed in U.S. Pat. No. 3,216,789 and is described as having the following composition in terms of moles of oxides:0.9-1.3M2/nO:Al2O3:5.2-6.9SiO2:yH2Owhere M is an exchangeable cation of valence n and y is from 0 to 9.
Methods for preparing zeolite L in the absence of an organic structure directing agent from a synthesis mixture containing sources of alkali metal ions, an oxide of silicon, an oxide of aluminum, and water are known. See, for example, U.S. Pat. Nos. 3,216,789; 3,867,512; 4,544,539; 4,657,749; 4,701,315; 4,894,214; 4,956,166; 5,017,353; 5,051,387; 5,064,630; 5,242,675; 5,318,766; 5,330,736; 5,491,119; and 5,670,130; U.S. Patent Application Publication No. 2013/0245349; and Great Britain Patent No. 1,202,511.
K. Honda et al. (J. Nanosci. Nanotechnol. 2013, 13, 3020-3026) disclose the preparation of a LTL framework type zeolite in the absence of an organic structure directing agent by the hydrothermal conversion of a FAU framework type zeolite having a Si/Al mole ratio of 25 at 125° C.
Conventional synthesis of LTL framework type zeolites yield cylindrical crystals with one-dimensional channels oriented along the c-axis length of the cylinder. Researchers have used various surfactants or so-called “molecular modifiers” to prepare LTL framework type zeolites with tunable morphology (see, e.g., A. I. Lupulescu, et al., J. Am. Chem. Soc. 2013, 135, 6608-6617).
It is known that crystal morphology, size and aggregation/agglomerization can affect catalyst behavior, especially regarding catalyst activity and stability. To improve mass transportation rates, zeolite crystals with small size can be desirable, e.g., for reducing diffusion limitations.
There is, therefore, a need for novel crystalline LTL framework type zeolite compositions and methods of making such compositions, especially zeolites of small crystal size and non-cylindrical morphology.